I'm not going to lie, even though I love my MAC, i am getting ready to install XP on it. I will be able to boot in either OS X or XP. The only reason I am doing so is software compatibility. The biggest problem I have is that Creative Labs doesn't make software for MACs, so i am stuck with old music until I find a legal copy of XP SP2, and i'm sure as hell not buying it.
Vista, in and of itself is a pretty darn good O/S, if you have the hardware for it...unfortunately application compatibility SUCKS, many reasonably new apps don't run correctly, and some won't run at all...
I have been advising any of my clients that are thinking about getting PC's in the next 12 months or so months to get them now so they can still get XP...I just moved my work laptop back to XP because I was tired of fighting compatibility issues